The Global Positioning System ("GPS") was conceived, developed, and deployed by the United States Department of Defense over ten years ago. The GPS' purpose was to provide air, ground, and naval users with precise position and time information. Although principally conceived for military purposes, the GPS also includes a band of signals designated for non-military use. These non-military signals may be used advantageously in numerous private and commercial applications. For example, the GPS can be used by commercial airlines and private pilots, to accurately and quickly ascertain their location. Similarly, both commercial shippers and private boatmen can use the GPS at any time to accurately determine their position, without the need for conventional points of reference, such as the magnetic poles.
The GPS is a series of 24 satellites orbiting the earth. The satellites are deployed in twelve hour orbits, and are dispersed in six orbital planes. These satellites continuously emit electronic signals--telemetry--which are received by ground based or other terrestrial receivers. By simply receiving the signal from four (4) or more orbiting GPS satellites, a properly configured receiver can accurately determine its position. This system has tremendous benefits over other positioning systems, in that it relies upon no visual, magnetic, or other point of reference. These advantages are particularly important in applications such as aviation and naval navigation which traverse polar regions where conventional magnetic navigational means are rendered less effective by local magnetic conditions.
The GPS system has many advantages over standard positioning systems which it replaces. The GPS is available 24 hours a day on a worldwide basis. There is no signal degradation, or loss of quality of positioning information at higher latitudes. Magnetic deviations and anomalies common in standard positioning systems do not exist. Typical GPS systems are fabricated of standard, solid state electronic hardware, resulting in a low cost, low maintenance system having few or no moving parts, and requiring no optics. The GPS does not require calibration, alignment, and maintenance such as conventional inertial measuring units. Further, normal GPS operation provides positioning with accuracies of about 100 meters for non-military uses. Military users can make use of encryption keys which yield position information within 10 meters.
Notwithstanding these advantages, there are still significant deficiencies in the capabilities of the GPS. For example, in naval applications, the GPS is incapable of providing information related to attitude, direction of travel, roll, and pitch. It may be appreciated that knowing that a ship is within 100 meters from running aground does not do any good if the direction of travel and attitude of the ship is unknown.
Prior art attempts to provide for attitude information in the GPS have encountered problems, principally phase measurement error. This results in degraded attitude/azimuth accuracy. Phase measurement error and attitude accuracy can be represented by the following equation: ##EQU1## where
PE=phase error [mm]
L=baseline length [mm]
AZDOP (Azimuthal Dilution of Precision) is a figure of merit used to quantify the quality of information received from a set of satellites (AZDOP) due to satellite geometry. The position of the satellites has a large effect on the attitude accuracy. EQU AZDOP=.sqroot.cos.sup.2 (az)*D.sub.11 +sin.sup.2 (az)*D.sub.22
where the matrix EQU D=(RA.sup.T AR.sup.T).sup.-1
and
az=azimuth angle
D.sub.ii =the iith element of the D matrix
A=double difference matrix operator which ensures independence in R
R=matrix of receiver to satellite vectors
The phase error factor (PE) is the sum of several phase error inducing mechanisms. It can be expanded as follows: EQU PE=ADPE+RPME+MP+EPL
where
ADPE=antenna differential phase error due to antenna phase center migration and signal interaction with the baseplane. PA1 EPL=electrical path length difference PA1 RPME=the GPS receiver phase measurement error (the accuracy to which the receiver can measure phase). PA1 MP=multipath (the reception of signals reflected off of objects in the antennas' environment) induced phase errors
The GPS' inability to provide attitude related information, combined with accuracy problems attributable to, for example, phase measurement error, severely limits the GPS' utility. These deficiencies and other limitations of the GPS system are addressed by the invention disclosed herein. These and other objects and advantages of the subject invention will become apparent from a perusal of the Detailed Description of the Invention, the Drawings, and the claims which follow.